This brief makes it sound like the second the timer hits zero and XP support ends, the lights will go out and planes will crash. That's not the way software support works. This will not suddenly render all XP machines inoperative. They will slowly become outdated, less functional, more vulnarable: exactly as you'd expect from not installing updates, no more.
I agree that XP has had a good run, much more than most operating systems get, and it's time for it to die, but to say that Microsoft's discontinuing of OS updates will "leave millions of existing Windows-based computers vulnerable to continued and undeterred cyberattacks" is just misleading.
I think the far more significant implication of this is the unspoken permission it gives web developers to stop supporting IE6. Which is probably cause for celebration.